Such a burner is described in the European Pat. No. 0 136 522, and corresponding U.S. Pat. No. 4,421,475, Frick. A distinction is made between atomizing burners and gasification burners. Atomizing burners atomize the fuel with a nozzle, and combustion takes place in a combustion chamber into which air is supplied. Because the amount of fuel atomized by a nozzle can only be varied within close limits, atomizing burners have the disadvantage that their heat supply cannot be continuously controlled. They cannot be built for very small heat requirements. The smallest nozzles are dimensioned for an oil throughput of approximately 1.4 kilograms per hour. Because the throughput of atomizing burners cannot be continuously varied, atomizing burners are intermittently operated when the heat requirement is small. Because the operating intervals cannot be made arbitrarily short, relatively large boilers are required as heat storage means. The intermittent operation has the disadvantage that repeated switching-on-and-off of the burner causes strong temperature change strains of the material and a high load of soot and noxious substances for boiler, chimney, and environments. Incomplete combustion and generation of soot occuring in particular during the start phase are detrimental to the overall efficiency of a heating system. In addition radiation losses of large boilers further diminish the total efficiency.
In contrast to atomizing burners gasification burners have generally the advantage that they can be regulated according to the heat requirements down to very small heating rates. In addition, combustion of gasified fuel provides a substantial decrease of the emission of noxious substances, e.g. unburned hydrocarbons, and soot.
In spite of the many advantages of gasification burners not much use of them is made. One reason for this is the fact that most gasification burners require much maintenance. Gasification burners generally tend to generate undesired deposits in the gasification chamber. These deposits quickly have a detrimental influence on gasification and therefore the operation of the burner.
In the European Pat. No. 0 036 128 a gasification burner with an electrically heatable gasification chamber is described. The temperature of the gasification chamber is measured by a temperature sensor and is kept by a control device on a optimal value to prevent carbonisation of fuel. A further feature to prevent carbonisation consists in that no air inlets into the gasification chamber are provided. In addition a rotatable devide in form of a wiper is located in the gasification chamber. This wiper serves to distribute the fuel evenly on the heated walls of the gasification chamber and to prevent the formation of deposits so that no detrimental influence of deposits on the evaporation of the fuel can occur. The gas formed in the gasification chamber exits through a nozzle with a relatively high velocity. The air required for combustion is provided by a fan. A modified form of this gasification burner is described in the aforementioned European Pat. No. 0 136 522, Frick.
The burners described in these citations have the disadvantage that they require relatively much electrical energy for the evaporation of the fuel. Burners of this kind are also relatively expensive, because they require a temperature sensor and a temperature control. Compared with other gasification burners where a mixture of fuel and air takes place in the gasification chamber prior to combustion, the combustion of the gas flowing with a relatively high velocity from a nozzle has the disadvantage that relatively high noises are generated. In addition, cold start problems may occur, because the air is not or only little heated prior to combustion. Further it is of disadvantage that on switchingoff smoldering of gasified fuel with a sooty flame may take place, if no costly measures have been taken to prevent the further release of gasified fuel from the pressurised gasification chamber.
The European Pat. No. 0 067 271, Noack, discloses an oil burner which can be continuously controlled. It is provided with an electrically heated evaporation device controlled by a thermostat. The evaporation device has the form of a beaker, the bottom of the beaker being provided with air inlet openings. In the beaker is a rotatable cylinder for the distribution of fuel. This cylinder fully occupies the evaporation chamber of the beaker with the exception of a small slot. To distribute oil, oil is fed into the rotating cylinder through a hollow drive shaft. The oil is then ejected through radial bores in the rotating cylinder to the inner walls of the evaporation chamber. However, oil burners of this kind have not found commercial application. Of disadvantage is that the evaporation chamber tends to become dirty. This hinders the entry of air and the exit of the mixture of air and gas. Because the pressure difference between air inlet and the outlet of the air/gas mixture is very small, a small amount of dirt is already sufficient to cause a sooty flame. A further disadvantage consists in that much heat is transferred to the surface of the rotating cylinder and further transferred over the drive shaft to the drive motor. Accordingly, the drive motor can be damaged if not costly devices for its protection are provided. The necessity of thermostat control of the evaporator means further increases the cost of the burner.
The U.S. Pat. No. 3 640 673, Okamoto, describes a burner for a kerosene stove on which a fan is located in the gasification chamber which is heated electrically or by the flame of the burner. Between the periphery of the fan and the heated walls of the gasification chamber there is a relatively large space. On the drive shaft for the fan is an atomizer plate for the fuel. If in operation fuel is directed on the atomizer plate, the fuel is distributed into fine droplets which are splashed outwardly. Thereby the droplets are mixed by the ventilator with the heated air flowing into the gasification chamber. Because the distance between the periphery of the fan and the heated walls of the gasification chamber is relatively large, most fuel droplets are evaporated without getting into contact with a wall. The few droplets which are hitting the heated wall are evaporated there. It is a disadvantage that at the walls deposits are formed which are detrimental to the evaporation, in particular during the start-up, when the gasification chamber is only electrically heated. This may cause start problems. A further disadvantage of the burner consists in that it is practically an atmospheric burner and therefore not suitable for use on a boiler.
In the European application No. 0 166 329, Fullemann, which was published on Jan. 2, 1986, a rotor provided with blades is described. The blades of this rotor extend close to the heated walls of the gasification chamber. The gasification chamber has an air inlet. The fuel fed through the hollow rotor shaft is finely distributed by the rotor and mixed with compressed air. In this way the fuel is evaporated in the hot gasification chamber. The mixture can then exit with relatively high pressure through the openings of a burner plate and burn with a practically noise-free blue flame.
For the sake of completeness it is referred to the oil burner described in the Swiss Pat. No. 628 724, Buschulte. This burner is an atomizing burner but also has features of a gasification burner. However, it has the disadvantage of atomizing burners that it cannot be regulated according to the heating requirements. Also at the lowest throughput it still requires 1.2 to 2.1 kilogram fuel per hour. To provide evaporation of the atomized fuel droplets, a mixing tube and a flame tube are located coaxially to the nozzle. In operation the fuel is sprayed by a nozzle into the mixing tube into which also the air necessary for combustion is fed. At the end of the mixing tube a flame is formed. A part of the hot combustion gases is then recirculated to the beginning of the mixing tube where it is mixed with the fuel mist/air mixture for the purpose of exchanging heat. Thanks to recirculation of a part of the combustion gases this burner permits a substantial evaporation of the oil droplets in the mixing tubes and therefore a better combustion with lower soot formation. However, as already been mentioned, this burner cannot be regulated within a wide range according to the heating requirements and requires in the lower range a still relatively high fuel throughput. Additional problems are caused during the start and the shut-off. This is still more detrimental, because the burner must be intermittently operated. During the start the mixing tube is cold and therefore has no evaporating action. Therefore the flame is very sooty until the mixing tube reaches a high temperature and is in a position to evaporate the arriving fuel. When the burner is switched-off a smoldering with a very sooty flame takes place of the oil dropping from the nozzle. Because on switching-off the mixing tube close to the nozzle is still glowing red, it radiates much heat against the nozzle which may cause carbonisation of fuel in the nozzle. This may cause the nozzle, particularly if it is a small nozzle, to clog.